Changes in the misfit stresses in an Al/SiCp metal matrix composite under plastic strain

Results are presented from neutron diffraction measurement of the strains in each phase, matrix and reinforcement, of a metal matrix composite bar before and after deformation beyond the elastic limit by four-point bending. The strains in each phase have been converted to stress. A stress separation technique was then applied, and the contributing mechanisms separated and identified. In this way the changes in the different contributions owing to plastic deformation have been determined. It is found that, initially, the average phase stresses can be explained in terms of a combination of essentially hydrostatic phase average thermal misfit stresses in the matrix (tension) and particles (compression) combined with a parabolic macrostress from quenching. After plastic bending the change in axial macrostress is as expected for that for a monolithic bar, but unexpectedly the misfit stresses had relaxed to approximately zero in both the tensile and compressive plastically strained regions of the bar.     

Effect of thermal stresses on the thermal expansion and damping behavior of ZA-27/aluminite metal matrix composites

When the fabrication of a metal matrix composite (MMC) involves its cooling from a high temperature, plastic-elastic residual deformation fields can be generated within and around the particle due to the differential thermal expansion between the particle and matrix metal. The present investigation is concerned with the effect of thermal residual stresses on the thermal expansion and damping behavior of aluminite particulate-reinforced ZA-27 alloy MMCs. Composites were prepared by the compocasting technique with 1, 2, 3, and 4 wt.% of aluminite reinforcement. Thermal expansion and damping properties have been studied experimentally as a function of temperature over a temperature range 30 to 300 °C both in the heating and cooling cycle. The thermal expansion studies exhibited some residual strain, which increased with the increase in the weight percent of the reinforcement. The damping capacity of both the composites and matrix alloy is found to increase with the increase in temperature during the heating cycle, whereas in the cooling cycle, damping behavior exhibits a maximum, which becomes more pronounced with the increase in the weight percentage of the reinforcement. The appearance of the maximum may be linked with dislocation generation and motion as a result of plastic deformation of the matrix at the metal/reinforcement interface. This phenomenon is attributed to the thermal stresses generated as a result of coefficient of thermal expansion (CTE) mismatch between the composite constituent phases. The thermal stresses have been estimated in both the cases using simple models.     

Use of neutron diffraction for stress measurements in thin and thick thermal sprayed coatings

Abstract

Thermal spraying is a widely used and cost effective technique for the surface protection of engineering components. The spectrum of applications is vast: corrosion protection, wear resistance and abrasion resistance, thermal barriers, electrical (dielectric) coatings, etc. Process induced residual stress has long been recognised as an important factor influencing the integrity and overall performance of coatings. Residual stress generation during thermal spraying is a complex phenomenon. Significant efforts have been made to improve understanding of the evolution of residual stresses during deposition and to develop practical models for numerical prediction of stress distributions in coatings. Owing to the high penetrating power of neutrons and spatial resolution in the millimetre and submillimetre range, neutron diffraction is, perhaps, the most versatile method for stress determination, and has been used extensively for experimental validation of theoretical predictions. Examples of neutron diffraction residual stress results are presented to illustrate the capabilities of the technique: a thin (～0·3 mm) Mo/Mo₂C composite HVOF coating, several examples of millimetre thick ceramic and metallic coatings, and thick coatings (～10 mm) of iron made by spray forming.     

Effect of thermal residual stresses on yielding behavior under tensile or compressive loading of short fiber reinforced metal matrix composite

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Quantifying residual stresses in resistance spot welding of 6061-T6 aluminum alloy sheets via neutron diffraction measurements

Residual strains of resistance spot welded joints of 6061-T6 aluminum alloy sheets were measured in three different directions denoted as in-plane longitudinal (σ₁₁), in-plane transversal (σ₂₂), and normal (σ₃₃). The welding process parameters were established to meet or exceed MIL-W-6858D specifications (i.e., approximately 5.7 mm weld nugget and minimum shearing force of 3.8 kN per weld confirmed via quasi-static tensile testing). Electron backscatter diffraction (EBSD) and optical microscopy (OM) were performed to determine grain size and orientation. The residual stress measurements were taken at a series of points along the weld centerline at depths corresponding to the weld mid-plane and at both 1 mm below the top surface of the plate and 1 mm above bottom surface. The residual stresses were captured on the fusion zone (FZ), heat affected zone (HAZ) and base metal (BM) of the resistance spot welded joint. Neutron diffraction results show residual stresses in the weld are approximately 40% lower than yield strength of the parent material. The maximum variation in residual stresses occurs, as expected, in the vertical position of the specimen because of the orientation of electrode clamping forces that produce a non-uniform solidification pattern. Despite the high anisotropy of the welding nugget and surrounding area, a significant result is that σ₃₃ measured stress values are negligible in both the horizontal and vertical directions of the specimen. Consequently, microstructure–property relationships characterized here can indeed inform continuum material models for application in multiscale models.     

金属基复合材料界面性能对残余应力的影响

采用ANSYS有限元软件计算涂层法制备的SCS-6 SiC/Ti-6Al-4V复合材料内热残余应力,分析了界面性能对热残余应力的影响。结果表明:较高的热膨胀系数(CTE)导致界面层产生高的应力梯度,使环向残余应力由低热膨胀系数时的压应力转变为较高的热膨胀系数时的拉应力;界面层弹性模量的增加,使得纤维和界面层内径向残余压应力明显增加,但对基体中的残余应力影响并不大;界面层厚度的变化对基体中径向残余应力影响不大,但随着界面层厚度增加,基体中残余应力有所减小。     

20 Hz synchrotron X-ray diffraction analysis in laser-pulsed WC-Co hard metal reveals oscillatory stresses and reversible composite plastification

The lifetime of WC-Co inserts used in cutting processes, such as milling, is limited by millisecond temperature and mechanical pulses, which occur as a result of interrupted tool-workpiece contact, thermal fatigue and wear. In the current work, synchrotron X-ray diffraction (XRD) was used in conjunction with a pulsed laser heating set-up to characterise the time-dependent development of stresses and microstructure in locally irradiated WC-Co inserts coated by chemical vapour deposition with 6.5 and 3.5 μm thick TiCN and α-Al₂O₃ films, respectively. Diffraction data from the WC phase were used to evaluate the time and temperature-dependent evolution of in-plane stresses, thermal strains and integral breadths of WC diffraction peaks in experiments with a single and five successive laser shocks applied within 2.2 and 20 s, respectively, using a laser spot diameter of ~5.8 mm and an X-ray beam size of 1 × 1 mm². The laser heating induces the formation of compressive stresses in the inserts' substrates. Above a temperature of ~750 °C, at the onset of WC-Co composite plastification, compressive stresses relax and then vanish in WC at the maximal applied temperature of ~1300 °C, followed by the build-up of tensile stresses. The applied cyclic heating up and cooling down led to the repetitive formation of compressive and tensile stresses, with temperature dependencies oscillating with the number of applied laser pulses. The observed relatively high tensile stress level of ~1100 MPa in WC was a consequence of the stabilising function of the coating, which hindered the initiation of surface hot cracks and stress relaxation. The stress evolution was coupled with changes in XRD peak broadening, which however strongly depended on the particular hkl reflections and showed oscillatory behaviour within a single temperature cycle. In summary, the unique diffraction set-up revealed stress levels and provides insight into the WC-Co composite plastification mechanism governing the stress build-up and relaxation in locally thermo-shocked WC-Co inserts at millisecond time resolution.     

A neutron diffraction study of lattice distortion,mismatch and misorientation in a single-crystal superalloy after different heat treatments

Two-dimensional neutron diffraction measurements of superlattice and fundamental lattice reflections from a single-crystal superalloy confirm the existence of angular distortion and shear stress in the γ′ phase, and reveal its correlation with the morphology and deformation of the γ′ phase during heat treatment. The tetragonal distortions with c/a > 1 are developed and retained during subsequent heat treatments, whereas the angular distortions are enhanced for the γ′ phase during the aging treatments. The evolution of the lattice mismatch and anisotropy during the subsequent heat treatments are consistent with the transformation of certain macroscopic anisotropic compositional and stress distributions in the as-cast sample to the microscopic level at the γ/γ′-interface upon reprecipitation and the growth of the γ′ from the supersaturated γ-matrix. The high negative γ/γ′ lattice mismatch of the alloy is consistent with the high levels of refractory elements, in particular Cr, in the alloy. Comparison between superlattice and fundamental lattice reflections is revealing. Firstly, the existence of lattice misorientation at the γ/γ′-interface and the discrete misorientations from the splitting of coherently aligned aggregates of γ′-precipitates on the smoothly distributed γ -matrix is shown. Secondly, the measured lattice misorientation correlates with the spacing of dislocations at the γ/γ′-interface and evidenced by the transmission electron microscopy observation. Furthermore, it indicates the anisotropic distribution of dendrites along crystal growth axis and the presence of small misoriented higher-order dendrite arms in variously heat-treated samples.     

Effect of thermal shield and gas flow on thermal elastic stresses in 300 mm silicon crystal

The thermal elastic stresses induced in 300 mm Si crystal may be great troubles because it can incur the generation of dislocations and undesirable excessive residual stresses.A special thermal modeling tool, CrysVUn, was used for numerical analysis of thermal elastic stresses and stress distribution of 300 mm Si crystal under the consideration of different thermal shields and gas flow conditions.The adopted governing partial equations for stress calculation are Cauchy's first and second laws of motion.It is demonstrated that the presence and shape of thermal shield, the gas pressure and velocity can strongly affect von Mises stress distribution in Si crystal.With steep-wall shield, however, the maximal stress and ratio of high stress area are relatively low.With slope-wall shield or without shield, both maximal stress and ratio of high stress area are increased in evidence.Whether thermal shields are used or not, the increase of gas flow velocity could raise the stress level.In contrast, the increase of gas pressure cannot result in so significant effect.The influence of thermal shield and gas flow should be attributed to the modification of heat conduction and heat radiation by them.     

Lattice stability,elastic constants and macroscopic moduli of NiTi martensites from first principles

The elastic constants that describe the fundamental elastic properties of NiTi martensites are unknown today. We present results of ab initio calculations of the ground-state energies and the relative mechanical stability of B19, B19′ and B33 (a theoretically predicted ground state from recent ab initio studies). It is demonstrated that shear stresses of the order of 1 GPa are sufficient to mechanically stabilize B19′ against B33. The full sets of elastic constants and the associated macroscopic elastic parameters (Young’s, shear and bulk moduli, Poisson ratios) are determined for the first time for B19′ and B33 NiTi. The predicted macroscopic Young’s modulus of B19′ based on the first-principles results is an order of magnitude larger than the values currently assumed in micro or continuum mechanical modeling studies. Yet the results are in good agreement with novel experimental data and, furthermore, resolve a long-standing issue in the well-known Müller–Achenbach–Seelecke model by predicting Young’s modulus of martensite to be larger than that of austenite.     

Evidence of variation in slip mode in a polycrystalline nickel-base superalloy with change in temperature from neutron diffraction strain measurements

The deformation mechanisms under tensile loading in a 45 vol.% γ′ polycrystalline nickel-base superalloy have been studied using neutron diffraction at 20 °C, 400 °C, 500 °C, 650 °C and 750 °C with the results interpreted via (self-consistent) polycrystal deformation modelling. The data demonstrate that such experiments are suited to detecting changes of the γ′ slip mode from {1 1 1} to {1 0 0} with increasing temperature. Between room temperature and 500 °C there is load transfer from γ′ to γ, indicating that γ′ is the softer phase. At higher temperatures, opposite load transfer is observed indicating that the γ matrix is softer. At 400 °C and 500 °C, an instantaneous yielding increment of about 2% was observed, after an initial strain of 1.5%. This instantaneous straining coincided with zero lattice misfit between γ and γ′ in the axial direction. Predicted and experimental results of the elastic strain response of the two phases and different grain families showed good agreement at elevated temperatures, while only qualitative agreement was found at 20 °C.     

Determination of elastic parameters in isotropic plates by using acoustic microscopy measurements and an optimization method

The determination of elastic constants in isotropic solids by line focus acoustic (LFA) microscopy is discussed. A velocity corresponding to a global resonance of leaky Lamb waves has been obtained for different frequencies of excitation (f) from the V(z,f) curves. This fact is analysed by using a Kushibiki spectrum analysis. The elastic constants are determined from the comparison between measured and calculated velocities. The principle is based on the minimization of the summation of the square difference ∑|V_f^c−V_f^m|² by using an algorithm based on the simplex method. This paper deals with the case of isotropic plates such as glass and aluminium plates. The influence of the initial values of c_ij on the final result is discussed in these cases. Experimental velocity profiles are measured on glass and aluminium plates as a function of the frequency varying from 8 to 17 MHz which corresponds to the bandwidth of the used transducer. The problem of geometric V_G(z) and measured V(z) curves fitting is also discussed.     

Through-thickness distributions of residual stresses in two extreme heat-input thick welds: A neutron diffraction,contour method and deep hole drilling study

Spatial variations of residual stresses were determined through the thickness of 70 mm thick ferritic steel welds created using low (1.7 kJ mm^?1) and high (56 kJ mm^?1) heat inputs. Two-dimensional maps of the longitudinal residual stress were obtained by using the contour method. The results were compared to neutron diffraction measurements through the thickness at different locations from the weld centerline. The deep hole drilling technique was utilized to confirm the maximum stress locations and magnitudes. The results show that significant tensile stresses (～90% of yield strength) occur along the weld centerline near the top surface (within 10% of the depth) in the low heat-input specimen. Meanwhile, in the high heat-input weld, the peak stress moved towards the heat-affected zone at a depth of ～40% of the thickness. Finally, the influence of residual stresses on potential fracture behavior of the welded joints is discussed.     

Measurement and prediction of residual stresses and crystallographic texture development in rolled Zircaloy-4 plates: X-ray diffraction and the self-consistent model

Complementary methods were used to analyse residual stresses and texture evolution in Zircaloy-4 sheets which had undergone cold-rolling deformation: X-ray diffraction and the self-consistent model. A modified elastoplastic self-consistent model, adapted to large deformation, was used to simulate the experimental results and showed close agreement with the experimental data. A new formulation of crystal plasticity is proposed. The influence and the role of elastoplastic anisotropy were also studied and explained in this work. Good agreement was found between experimental and predicted crystallographic textures. The contribution and the magnitude of the first- and second-order residual stresses were correctly evaluated using information from the model. Comparison between the X-ray diffraction results and the simulations confirms that prismatic slip is the main active deformation mode in this alloy under large strain.     

Determination of internal stresses in cyclically deformed copper single crystals using convergent-beam electron diffraction and dislocation dipole separation measurements

Single crystals of copper were cyclically deformed, in single slip, to presaturation at 298 K. The dislocation substructure was analyzed using conventional bright-field and dark-field transmission electron microscopy with particular attention directed towards the dislocation dipole spacing. It was found that, in both metals, the dipole spacing and statistical distribution of spacings were independent of the location in the heterogeneous substructure, which consisted of dense dipole bundles (or veins) and channels with relatively low dislocation density. Furthermore, the stress to separate the dipoles with largest spacing (upper-bound separation for stable dipoles) can be used to calculate the stress at the dipole location. This stress is within a factor of about two of the applied stress in both channels and veins. The stress necessary to pass dislocations through the dense veins was calculated to also be within a factor of about two of the applied stress. Convergent-beam electron diffraction (CBED) experiments were also performed at several locations very near the dipole bundles and within the channels. The lattice parameter measurements also suggested an absence of long-range internal stresses. The observations and calculations suggest a uniform state of stress throughout the heterogeneous dislocation substructure, without the presence of significant internal stresses.     

Formation process, microstructure and mechanical property of transient liquid phase bonded aluminium-based metal matrix composite joint

1　INTRODUCTIONThealuminium basedmetalmatrixcomposites(MMCs)areadvancedmaterialsthathavesuperiorproperties ,especiallyincreasedstiffness ,highstrength ,goodwearresistanceandsuperiorelevatedtemperatureproperties .Theyhavereceivedconsider ableattentionascandidatesforadvancedindustrialapplications[1,2 ] .But ,theirapplicationshavebeenseverelyrestrictedbythelackofasuitablejoiningmethod[3] .AlthougthfusionweldingmethodscanbeusedtojointheMMCs ,themethodsnormallytendtoresultinunfavourablejoint…     

A unified interpretation of threshold stresses in the creep and high strain rate superplasticity of metal matrix composites

The flow behavior of metal matrix composites is characterized by the presence of a threshold stress under both creep conditions at intermediate temperatures and in high strain rate superplasticity (HSR SP) at very high temperatures near the onset of partial melting. Experiments show the measured threshold stresses decrease with increasing temperature and this trend has been interpreted using an Arrhenius-type relationship incorporating an energy term, Q₀. Typically, the experimental values reported for Q₀ are ∼20–30 kJ/mol under creep conditions but up to ∼100 kJ/mol in experiments associated with HSR SP. This report resolves this apparent dichotomy by demonstrating that both sets of results become consistent when the analysis is extended to incorporate an additional dependence on temperature associated with load transfer and substructure strengthening.     

Characterisation of void and reinforcement distributions in a metal matrix composite by X-ray edge-contrast microtomography

Dual energy X-ray microtomography measurements above and below the Zr K absorption edge have been used to make separate reconstructions of ZrO₂ particle and void distributions in failed metal matrix composite tensile test specimens. A correlation is found between the local concentration of voids nucleated prior to failure and the local concentration of reinforcement.